22 months ago my father was diagnosed with prostate cancer. He had brachy therapy and hormonal treatment. In January his PSA count was 6.4, now it is 130. the cancer has spread to his bones and he is unable to walk. He sleeps most of the day, and eats VERY little. He has also deteriorated mentally to a stage where he is unable to construct sentences or remember what happened five minutes ago. He still remembers closely related people but sometimes forgets their names.
I was wondering if anyone could shed some light on why his cognitive state is so badly deteriorated and what we can expect next as he is getting worse every second day.

He should be on hospice as his time is short. You can expect this to continue to get worse, for him to sleep more, eat less, his mental status may not return to normal at all, he will not react to stimuli and in general the disease will progress until he dies.

Hopefully he is just tired and not in pain as bone mets can be very painful, you want everything in place just in case so that he does not suffer a minute longer than necessary.

There must be something more than cancer in the prostate and bones. Maybe he has Altzheimer's? Brain function should not be affected by the cancer unless it has spread to the brain or to some other organ that is depriving his body of the chemicals needed for normal brain function. The brain can also get pretty foggy if he is not getting enough food. I don't think you can know what to expect until you find out exactly what's causing the mental deterioration.